Recently, as a rewritable nonvolatile memory, the magnetic random access memory (hereinafter called MRAM) including magnetoresistive effect elements arranged in a matrix is noted. The MRAM memorizes information by using combinations of magnetization directions of the magnetic layers and reads memorized information by detecting resistance changes (i.e., current changes or voltage changes) between the parallel magnetization directions of the magnetic layers and the anti-parallel magnetization directions of the magnetic layers.
As one of the magnetoresistive effect elements forming the MRAM is known the magnetic tunnel junction (Hereinafter called MTJ) element. The MTJ element includes two ferromagnetic layers stacked with a tunnel insulating film formed therebetween and utilizes the phenomenon that the tunneling current flowing between the magnetic layers via the tunnel insulating film changes based on relationships of the magnetization directions of the two ferromagnetic layers. That is, the MTJ element has low element resistance when the magnetization directions of the two ferromagnetic layers is parallel with each other, and when the magnetization directions of the two ferromagnetic layers are anti-parallel with each other, has high element resistance. These two states are related to data “0” and data “1” to be used as the memory device.
As memory cells of the conventional MRAM are known the so-called 2T-2MTJ type MRAM comprising memory cells each including two select transistors and two MTJ elements, and the so-called 1T-1MTJ type MRAM comprising memory cells each including one select transistor and one MTJ element.
In the 2T-2MTJ type MRAM, information complementary to each other is written into two MTJ elements, and the information memorized in the memory cells is read by comparing to judge which MTJ elements have the high resistance state and the low resistance state. The 2T-2MTJ type MRAM is superior in the stability of the read operation, and at the start of the development of the MRAM, the 2T-2MTJ type MRAM was the major memory cell structure. However, the 2T-2MTJ type MRAM has the memory cells each including many constituent elements, which is disadvantageous in the integration. In the field of memories, it is usual to compare the areas of the memory cells with the minimum feature size being F. Based on this index, the memory cell size of the 2T-2MTJ type MRAM is 16 F2.
The 1T-1MTJ type MRAM is a memory cell structure proposed as a structure which can be more integrated. The number of the constituent elements of one memory cell of the 1T-1MTJ type MRAM is a half the element number of the 2T-2MTJ type MRAM. This is advantageous in the integration. Based on the index described above, the memory cell size of the 1T-1MTJ type MRAM is 8 F2. In the 1T-1MTJ type MRAM, however, memorized information is read by the comparison with a reference signal potential, which makes the operation margin smaller than the 2T-2MTJ type MRAM.
As memory cell structures which can be further integrated, the so-called NAND-type circuit MRAM (hereinafter called NAND-type MRAM) is disclosed in, e.g., Japanese published unexamined patent application No. 2004-200641, Japanese published unexamined patent application No. 2004-213744, and U.S. patent unexamined publication No. 2002/0097598. As shown in FIG. 1, the NAND-type MRAM includes a unit cell block 200 including a plurality of serially connected 1T-1MTJ type memory cells (MC) each including a select transistor (Tr) and an MTJ element (MTJ) parallelly connected. The unit cell block 200 has one end connected to a power supply line 202 via a cell block select transistor Trsel and has the other end connected to a read line 204. The NAND-type MRAM, which needs no contact region between each of the serially connected memory cells and its adjacent one, can be integrated more easily than the usual 1T-1MTJ type MRAM and, based on the above-described index, can be integrated up to about 4 F2.
However, the conventional magnetic memory devices have smaller read operation margins as the memory cell structures have higher integration of the memory cells.